1. Field of the Invention
The present invention relates to an optical proximity correction method. More particularly, the present invention relates to an optical proximity correction method for modifying the pattern on a photomask so that a higher photolithographic resolution is obtained.
2. Description of the Related Art
As the level of circuit integration continues to increase, dimensions of each electronic device must be reduced correspondingly. Photolithography is an important process in the fabrication of microelectronic devices on a silicon chip. Most structures associated with the fabrication metal-oxide-semiconductor (MOS) device such as patterned film layers and doped regions are closely related to the resolution of photolithographic processes. In fact, further increases in the level of circuit integration depend on our capacity to pattern out a line width smaller than 0.15 .mu.m in photolithographic processes. To fabricate devices with such a small line width, methods such as optical proximity correction (OPC) and phase shift mask (PSM) have been developed.
The purpose of performing optical proximity correction is to eliminate deviations in critical dimensions from a desired pattern due to proximity of pattern features. Proximity effect occurs when a light beam passes through the pattern of a photomask and projects onto a silicon chip. Due to the dispersion of the incoming light beam by the photomask, a portion of the light beam is diffused. Furthermore, some of the light will be reflected back from the surface of the photoresist layer, resulting in light interference. Consequently, over-exposure of light in some regions of the photoresist layer occurs, resulting in pattern distortion. This phenomenon becomes increasingly dominant as critical dimension decreases and wavelength of the light source approaches the critical dimension.
FIGS. 1A through 1D are schematic top views showing the steps for carrying out conventional optical proximity correction treatment. FIG. 1A is a top view of a photomask 100 showing an integrated circuit pattern with three rectangular masking regions 105 surrounded by a transparent region 110. Structurally, the photomask 100 consists of a chromium layer above a transparent substrate. Materials for forming the transparent substrate include glass and quartz. Area not covered by any chromium forms the transparent region 110, whereas areas covered by the chromium layer become the masking regions 105. FIG. 1B is a top view showing the resulting pattern when light is projected onto a substrate 120 through the photomask 100. A pattern with three dark regions 125 surrounded by a bright region 130 is formed. As shown in FIG. 1A, the original masking region 105 has a rectangular shape. However, the dark regions 125 on the substrate 120 transferred to a photoresist layer (as shown in FIG. 1B) display some shape distortion. Due to the dispersion or diffraction of light near the edges of the chromium layer, corners are smoothed into arcs in addition to a minor reduction of lateral dimensions. Moreover, other types of pattern distortions not shown in the figure are also possible. For example, when pattern density of the photomask is high, some features may merge, or alternatively, some features may deviate from the intended locations.
To compensate for the distortions, masking regions are sometimes expanded in laces next to the corners and edges of the masking region 105. FIG. 1C is a top view showing a photomask with added masking regions 150 and 155. The additional masking region 150 at a corner is called a serif. The serif is able to reduce the degree of arc formation in a pattern after photoresist exposure. The additional masking regions 155 are designed to reduce dimensional reduction due to diffraction or dispersion along the edges of a pattern. FIG. 1D is a top view of the pattern obtained on a substrate 120 after a photomask as shown in FIG. 1C is used in light projection. As shown in FIG. 1D, arcing at the corners of dark regions 125a has improved considerably.
However, the addition of masking areas to the masking pattern is not feasible when distance between neighboring features lines is small or critical dimension has fallen to below 0.15 .mu.m. A bottleneck is encountered because no more space is available for such compensation.
To form a photomask with higher resolution or a photomask with complicated features optical correction using the aforementioned method is difficult. Hence, correction must be carried out with the aid of a computer. In general, data concerning the desired pattern on a semiconductor substrate is first stored inside a computer, and then iterative computation is carried out using conventional computer software. Ultimately, an optimal mask pattern is obtained. The optimal mask pattern is stored inside a computer. Finally, the ideal pattern is used to fabricate the photomask. When a light beam is shone on the photomask, an image that closely resembles the intended pattern is projected onto the semiconductor substrate.
Although computer programming is able to improve resolution of photomask, it can do so only up to a certain limit. For a pattern with ultra-fine features, the desired resolution may exceed the capability of the optical proximity correction method. Moreover, time-consuming computations have to be executed every time some modification to the pattern is need. The amount of time spent in processing large volumes of data and the necessary inspection of photomasks make mass-production of integrated circuits inconvenient.